In general, the blood plasma disappearance rate and the retention rate have been measured by a method of blood collection through use of indocyanine green (hereinafter referred to as ICG) serving as specific dye. According to this method, an intravenous injection of ICG is given to a testee and blood collections are made three times after lapses of five, ten and fifteen minutes after the injection, and blood serum is separated upon coagulation of a blood clot so that an absorbance at a wavelength of 805 nm is measured through a spectrophotometer to obtain ICG concentration values in the blood serum after the lapses of five, ten and fifteen minutes from a previously obtained calibration curve representing an ICG concentration in blood as a function of absorbance. Thus, it is possible to calculate the blood plasma disappearance rate and the retention rate. In recent years, a method of changing the quantity of the ICG injection to measure the blood plasma disappearance rate several times has been widely used for obtaining an index expressing an amount of hepatic cell function R.sub.MAX (removal maximal).
Japanese Patent Publication Gazette No. 58649/1985 has already proposed a method of measuring the blood plasma disappearance rate and the retention rate without performing any blood collection. According to said method, light is applied through the body surface of an organism, which in turn transmits light of a wavelength having a high ICG absorption sensitivity and light of a wavelength having substantially no ICG absorption sensitivity. The respective quantities of transmitted light are measured to obtain the blood plasma disappearance rate and the retention rate as a function of elapsed time (dye disappearance curve) of the light quantities.
In the aforementioned first mentioned method requiring the collection of blood samples, it is necessary to correctly measure the blood collection time after injection. However, the time cannot be accurately measured in fact, and the practical measuring of the index expressing the amount of hepatic cell function R.sub.MAX has been complicated in its adaptation to the theory. Further, the testee has been subjected to heavy mental and physical burdens by the repeated taking of blood samples. In addition, the index R.sub.MAX method of measuring the blood plasma disappearance rate several times by changing the quantity of ICG injection requires the taking of more than ten blood samples, whereby the burdens on the testee are further increased.
According to the second mentioned measuring method which does not require the taking of any blood samples disclosed in Japanese Patent Publication Gazette No. 58649/1985, the output of a sensor actually attached to an organism, fluctuates under the influence of such facts as blood flow disturbances caused by a compression on a blood vessel, vibrations of the organism that is tested, pulsations in the organism, changes in the blood volume in the vital tissue. The blood volume in each part of a vital tissue changes due to movements, for example, by merely vertically moving an arm, etc., whereby a correct dye disappearance curve cannot be obtained. Consequently, the blood plasma disappearance rate and the retention rate obtained by the curve cannot be recognized as being correct.